1. Field of the invention
The present invention is generally concerned with the ultrasonic treatment of objects, surfaces or organisms, possibly making use of the phenomenon known as "ultrasonic cavitation".
2. Description of the prior art
As is known, and whatever the medium concerned, there is associated with a field of ultrasonic waves or vibrations a field of pressure waves which have a mechanical effect which can result in the destruction of micro-organisms, the mechanical vibration of particles and/or the instigation of resonance in particles, which can fragment or even destroy them.
In the case of a high intensity ultrasonic field in a liquid medium there occurs a phenomenon known as "cavitation" which results in destructive mechanical and chemical action with respect to micro-organisms. In the case of radioactive particles, the combination of these two actions can result in detachment of the particles and consequently in decontamination of the objects to which these radioactive particle were attached, subsequent to evacuation of such particles.
It is known that when a liquid is subjected to an alternating ultrasonic wave field the field causes variations in pressure within the liquid and so creates areas subjected alternately to decreases and increases of pressure. In an area of reduced pressure cavities are created and fill with gas. It is accepted that these cavities arise at microbubbles in suspension in the liquid or trapped at the surface of any kind of solid impurity. During successive phases of increased and reduced pressure brought about by the ultrasonic field, the diameters of these cavities or bubbles vary.
So-called vapor cavitation conditions are produced in the following way:
During the reduced pressure phase created by the ultrasonic wave the cavitation bubble grows to a maximum radius which may be in the order of 40 times the initial radius. In the subsequent increased pressure phase it is compressed and implodes suddenly, giving rise to numerous effects including:
the formation of a shock wave which propagates at a speed which may exceed that of sound in the liquid, creating in the vicinity very high pressure and temperature fields;
the occurrence of sonoluminescence;
the formation of particularly active ions (OH.sup.- and H.sup.+), releasing oxygen molecules and forming oxygenated water.
At the implosion sites gas bubbles form on the next cycle during the pressure reduction and the process resumes. Thus generations of bubbles vary in volume at the same point at a rate determined by the ultrasonic wave, giving rise to cavitation very quickly. Cavitation manifests itself by a characteristic whistling sound and a cloud of bubbles.
By virtue of the formation at the ultrasonic frequency used by turbulence associated with very high pressure and temperature fields, ultrasonic cavitation has a particularly intense eroding effect on surfaces disposed in the immediate vicinity of the cavitation area, and is consequently highly effective for cleaning such surfaces.
The mechanical and chemical phenomena mentioned hereinabove result in the destruction of bacteria that may be attached to the surfaces to be cleaned. Thus it is possible to consider ultrasonic cavitation as a means of cleaning and/or sterilizing surfaces.
The same applies in the case of radioactive deposits of dirt such as may be found on surfaces to be decontaminated such as those of nuclear reactor pools or within pipes in nuclear power plants. The mechanical and chemical phenomena to which the contaminated particles are subjected necessarily detach them from their support which, following evacuation of the particles, is decontaminated.
In the context of the present invention, the concept of cleaning is to be understood as encompassing that of decontamination. When the surfaces or objects to be cleaned are of significant dimensions, the effectiveness of a cleaning device of this kind is related to the volume of the liquid into which ultrasound can be injected, however. The problem of the quantity of energy available may then arise.
The present invention concerns a device for injecting ultrasound into a solid, liquid or gaseous medium which features especially high efficiency and energy dispersion capability. The present invention also concerns cleaning apparatus and installations employing this device.
There have previously been developed ultrasonic devices, more specifically for liquid media, which comprise at least one ultrasonic transducer, which is generally a piezo-electric (usually called "ceramic") cell, and at least one probe of which a so-called "active" or "cavitation" part is immersed in said medium and comprises a part for matching the impedance to said medium with an end face from which longitudinal ultrasonic waves are emitted. As a general rule, these devices comprise one or more conversion members, namely a "quarter-wavelength section" and one or more "sonotrodes". The impedance adapter consists of a set of such sonotrodes, which are members having a length equal to a multiple of half the wavelength in the material of which they are made at the excitation frequency and the cross-section of which varies, generally according to some form of hyperbolic function, constant or decreasing in the primary wave propagation direction. The sonotrodes multiply the amplitude of the vibrations at the frequency in question by the ratio of their inlet to outlet surface area.
Although the efficiency of an ultrasonic device of this kind is satisfactory in certain applications, it is relatively often insufficient in other applications requiring a considerable dispersion of energy. Energy is supplied to the medium only from the end surface which, given the characteristic shape of the terminal part of the impedance adapted, usually that of an inverted horn, is relatively limited.
In this type of implementation there is a lack of energy available for emission in the form of longitudinal vibrations and, whatever else may be the case, inadequate effectiveness when only these longitudinal vibrations are used.
Also known in the prior art are so-called ultrasonic tanks. These tanks are filled with a liquid, generally water to which a detergent has been added. The objects to be cleaned are then totally immersed in the tank. The water is subjected to an ultrasonic field by means of an ultrasonic device, of substantially the same type as that briefly described hereinabove, attached to the bottom of the tank. Cavitation arises in the liquid within the tank.
This type of tank generally proves satisfactory but its capabilities are limited by its internal dimensions, with the result that certain bulky objects cannot be cleaned in this way.
There are also known dental probes that the dentist holds in his hand. These probes provide for the emission of a relativly fine jet of liquid which is subjected to an ultrasonic field. Although it is well adapted to the problem of dental surgery, this technology may be difficult to extrapolate to the cleaning of large industrial objects. Generally speaking, this technique is characterized by a particularly low efficiency. Also, industrial objects to be cleaned are, as has already been said, often relatively bulky, which makes it difficult to extrapolate and adapt these dental probes.
An objective of the present invention is to propose an ultrasonic device offering particuarly high efficiency and providing for a high degree of energy dispersion whatever the medium excited by means of the ultrasonic field.
Another objective of the present invention is to propose various forms of cleaning apparatus and installation providing in particular for cleaning industrial objects of particularly large size and hollow objects the space inside which is accessible only with difficulty. Another application of the ultrasonic device in accordance with the present invention consists in apparatus and installations for cleaning surfaces such as tunnels, hospital rooms or nuclear reactor pools in electric power plants.